Many proposals have been made heretofore for improvements in tear-off end envelopes. One early example of such proposals is found in U.S. Pat. No. 211,725 issued Jan. 28, 1879 to Foster, wherein a generally rectilinear line of perforations adjacent an end of the envelope was provided.
In U.S. Pat. No. 3,294,313 issued Dec. 27, 1966 to Spaulding, it was recognized that the perforations permitted the end of the envelope to be bent and flexed on a rectilinear weakening line, whereby the envelope may be accidentaly opened. Spaulding therefore proposed a sinuous line of perforations to overcome this problem.
A further proposal for strengthening the weakened end is found in U.S. Pat. No. 1,098,175 issued May 26, 1914 to Schnitzler, wherein a reinforcing paper strip was adhered to interior surfaces of the envelope to bridge across the perforations. Both single layer and double layer strips were proposed. In that both the reinforcing strip and the envelope in this proposal are made of identical material, i.e. paper, such reinforcing strip appears to defeat the desired object of providing an easily opened tear-off end envelope.
In recent years, post offices have turned increasingly to the use of mechanized handling equipment, and it has become even more necessary to ensure the strength and integrity of envelopes consigned to the general mails. Nonetheless, it is desirable to retain the easy tear-off end feature of envelopes, and also to improve their construction.
The ease of opening of a tear-off end envelope will normally depend upon the closeness of the perforations. Indeed, in another type of envelope wherein the perforations are very closely located, the end may be pulled off with a snap-action. This type of construction is exemplified in FIG. 1 of U.S. Pat. No. 1,180,542, issued Apr. 25, 1916 to Roden, and is generally limited to applications where the contents of the envelope are united with the end of the envelope and are relatively thin, whereby the envelope is unlikely to be caught up in handling machinery. Where the perforations are widely spread apart, the tensile strength required to snap open the tear-off portion of the envelope will increase correspondingly, but the ease with which the end may be torn off will decrease, and the appearance of the body of the envelope from which the end is torn may be somewhat ragged, making the construction less desirable.
It is then, an object of this invention to provide a tear-off end envelope of enhanced strength and durability.
It is another object of this invention to provide a tear-off end envelope which may have a resistance to forces equal to that of the non-perforated end of the envelope, and yet which will tear open along a well formed straight edge.